Organophosphate flame retardants (OPFRs) persist in the environment and have been used in consumer products since the 1970s. Restrictions on the use of penta-polybrominated diphenyl ether (PBDE) flame retardants have resulted in increased use of OPFRs as replacements, including the OP triesters [tris(1,3- dichloro-2-propyl) phosphate (TDCPP), tris(chloropropyl) phosphate (TCPP), tris(2-chloroethyl) phosphate (TCEP), and triphenyl phosphate (TPP)]. These chemicals are applied to polyurethane foams used in furniture, child car seats, and related products to meet federal and state flammability standards. OPFRs have been measured in >95% of house dust samples collected in the United States, as well as in the foam of numerous baby products. In another recent study, these compounds were detected in >95% of urine samples from adults. Animal research suggests that OPFRs may affect neurodevelopment through non- cholinergic mechanisms similar to some organophosphate (OP) pesticides. Despite the widespread presence of these compounds in home environments, and their structural similarity to neurotoxic OP pesticides, understanding of human exposure and health effects of OP triesters is scarce. To date, no studies have examined OPFR exposures and health effects in vulnerable populations, such as pregnant women and children living in California, where stricter flammability standards have resulted in very high flame retardant exposures. Recently, we developed laboratory methods to measure metabolites of the high-use OPFR flame retardants TDCPP and TPP in urine. For this proposal, we will use specimens and data from the CHAMACOS study, an ongoing NIEHS-EPA funded birth cohort study of low-income residents in the Salinas Valley, CA. We propose to measure OPFR metabolites in mothers' urine and examine the association of prenatal OPFR exposure and neurodevelopment in their children at 7 years (n=330). We will also measure OPFR levels in dust samples collected from a subset of these CHAMACOS women's homes (n=125), determine the correlation between OPFRs in urine and dust, and identify predictors of home OPFR contamination. This will be the first study to: 1) evaluate predictors of OPFR contamination in homes; 2) report levels of biological measurements of exposure to OPFRs in pregnant women; and 3) study the relationship between in utero OPFR exposure and human neurodevelopment. This new information about human exposure to OPFRs will support the development of more comprehensive studies by applying innovative scientific methods to address key knowledge gaps about OPFR exposure and health effects.